J. Am. Chem. Soc.http://dx.doi.org/10.1021/ja208463f (2011)

Most miniaturized analytical assays carried out using surface-enhanced Raman scattering (SERS) in solution require the SERS-active gold nanoparticles (AuNPs) to first capture the analyte and then to be reconcentrated for sensitive detection. Now Keith Carron and colleagues describe how the scavenging and reconcentration steps can be combined. Functionalized AuNPs are immobilized onto the surface of a buoyant hollow silica microparticle, which functions as a floating device to direct the analyte to accumulate spontaneously and efficiently at the top of the aqueous sample. Described as a lab-on-a-bubble (LoB) assay, the method overcomes two significant limitations of low-volume SERS assays; it avoids the use of an inefficient and time-consuming reconcentration step, and eliminates the contribution of Brownian motion to the spectral noise (typically seen with traditional gold substrates) as the buoyant particles are significantly larger than the laser focus. Furthermore, because the immobilized AuNPs do not aggregate, there is no loss of SERS signal over time. Initial results from the LoB assay are promising: the limit of detection for cyanide ions is approximately 170 parts per trillion, which is on a par with other state-of-the-art detection systems.